1. Field of the Invention
The invention relates generally to seals for spectroscopic measuring devices and, more particularly, to a method and apparatus for sealing an optical window in a spectroscopic measuring device.
2. Description of Prior Art
In the chemical process industry, spectroscopic measuring devices are often used to analyze reactions. Such devices are often provided as specialized probes or as flow cells of minute proportions. They typically use ultraviolet to visible (xe2x80x9cUV-VISxe2x80x9d) or infrared (xe2x80x9cIRxe2x80x9d) radiation. As such, these devices typically include an optical element that separates the measuring device""s optical paths from the reaction while transmitting the necessary radiation. The optical elements, therefore, must have great mechanical strength, high resistance to chemical attack, and broad optical transmission range. A sapphire window is commonly used as the optical element commonly in such devices, but other materials may be sued where desired.
It is important, of course, to seal the optical element to its surrounding structure. The prior art consists of various methods of sealing the typically non-metal optical element to the typically metallic housing of the measuring device.
However, the prior art has inadequately dealt with the various environmental problems encountered in chemical processes that may cause the seal to fail, thereby rendering the measuring device inoperable and possibly harming nearby operators. These environmental problems include low temperatures, high temperatures, extreme temperature cycling, high pressure, fluctuations in pressure, and aggressive chemical reaction. Not only do chemical processes range in temperature from cryogenic to 500 degrees Celsius (xe2x80x9cCxe2x80x9d) or more, the rate of change in temperature may be up to 50 C. per minute. Such temperature cycling causes materials to expand and contract and may cause a conventional seal to leak. Temperature cycling is especially troublesome with materials having a high thermal coefficient of expansion and at junctions between materials having different coefficients of expansion. Changes in pressure can also affect the expansion of materials. Harsh chemical reactions attack and wear away the composition of a material. In addition, spectroscopic measuring devices, such as probes and flow cells, tend to be relatively small in size, thereby reducing the range and flexibility of dimensions and materials available in making a seal.
The prior art has attempted to address these environmental problems by using elastomeric seals, by brazing sapphire to metal, and by xe2x80x9csweatingxe2x80x9d the window directly to the metal housing. Each of these prior art methods and structures has its disadvantages when encountering either temperature cycling or aggressive chemical reaction.
A poplular elastomeric seal, for example, is a high performance perfluoroelastomer O-ring. Though highly chemically resistant, such O-rings may expand up to 20% at upper temperatures near 280-300 C. As such, frequent and broad temperature cycles may cause these elastomeric seals to leak. Moreover, repeated temperature cycling may cause them to harden. The same problems are encountered with filled polymer, usually Teflon based, metal spring energized seals.
Brazing consists of bonding sapphire to a metallic housing by metalizing the perimeter sealing surface of the sapphire with specialized materials and then brazing that surface to a metal housing. Since sapphire has a low thermal coefficient of expansion, it is likewise preferable to use a specialized material with a low thermal coefficient of expansion in order to provide a more reliable seal that is less likely to leak. However, a dilemma exists in that most low expansion metals have high iron content, thus making them susceptible to attack by chemical processes. A more chemically resistant metal, however, usually has a higher thermal coefficient of expansion.
Sweating consists of fitting the window directly in the metal housing without use of additional materials. However, the differing thermal coefficients of expansion between the sapphire and the parent metal, such as nickel alloy, may cause leaks to occur at or above 280 C.
There remains a need, therefore, for a spectroscopic measuring device having an improved seal structure.
In accordance with the present invention, both a structure and method are disclosed which overcome these obstacles. Structurally, the invention comprises both a spectroscopic measuring device as well as a seal for a spectroscopic measuring device.
The invention comprises a housing, a window, an intermediate ring, and a retaining ring. The housing comprises a first material and defines a cavity. The first material is preferably a metallic alloy such as nickel-molybdenum-chromium alloy. Disposed in the cavity, the window has a first portion adjacent to an exterior of the device and a second portion. The first portion has a first diameter and a first side wall. The second portion has a second diameter and a second side wall. The second diameter is larger than the first diameter such that a shoulder is formed between the second and first portion and a gap is defined between the first side wall and an interior surface of the housing.
The intermediate ring is disposed in the gap adjacent to the shoulder of the window. The intermediate ring comprises an annular C-ring or an O-ring and has a coating of material with a higher thermal coefficient of expansion than that of the ring itself. Such a coating may comprise gold plating, or silver, or platinum.
The retaining ring is disposed in the gap adjacent to the intermediate ring. The retaining ring has an exposed retaining surface that is flush with both an end housing surface of the housing and an exposed sapphire surface of the window. An outer juxtaposition is defined between the retaining ring and the housing while an inner juxtaposition is defined between the retaining ring and the window. A portion of the exposed retaining surface is electron beam welded to a portion of the end housing surface at the outer juxtaposition.
The retaining ring and the intermediate ring comprise the first material of the housing.
In an alternate embodiment, the exposed sapphire surface of the window is not flush with the exposed retaining surface of the retaining ring. Thus, only an outer juxtaposition is defined between the retaining ring and the housing. A portion of the exposed retaining surface is electron beam welded to a portion of the end housing surface at the outer juxtaposition.
In an alternate embodiment, a gasket is provided and displaced between the shoulder of the housing and the inner surface of the window.
The invention further comprises a method for sealing a spectroscopic measuring device 10. The method comprises the steps of: disposing a window 40 in a cavity 65 of a housing 60; disposing an intermediate ring, which may comprise a metallic C-ring 30 or O-ring 30a for example, in the cavity 65 adjacent to the window 40; disposing a retaining ring 20 in the cavity adjacent to an exterior of the device 10 and the intermediate ring 30 or 30a; and welding a portion of an exposed retaining surface 21 of the retaining ring 20 to a portion of an end housing surface 61 of the housing 60.
The preferred method involves pressing the retaining ring 20 into the cavity so as to crush the intermediate ring 30, 30a as appropriate. Preferably, until contact is made during an initial part of the press-fitting of the retaining ring 20, a thin tube is inserted between the outside diameter of the window""s first portion 42 and the inside diameter of the retaining ring 20 in order to center the intermediate ring 30, 30a and thereby provide a more reliable seal.
The method may further comprise the step of coating the intermediate ring 30 with a plating material having a higher thermal coefficient of expansion than a thermal coefficient of expansion of the intermediate ring, such as gold. The method also comprises the step of providing a shoulder 43 on the window 40, wherein the step of disposing the intermediate ring 30 or 30a in the cavity adjacent to the window 40 comprises the step of disposing the intermediate ring 30 or 30a adjacent to the shoulder 43 of the window 40.
The step of welding the portion of the exposed retaining surface 21 of the retaining ring 20 to the portion of the end housing surface 61 of the housing 60 preferably comprises the step of electron beam welding the portion of the exposed retaining surface 21 to the portion of the end housing surface 61. Other methods of sealing the two surfaces, however, may be used with equal applicability.
The step of disposing the retaining ring 20 in the cavity 65 adjacent to the exterior of the device 10 and the intermediate ring 30 may comprise the step of disposing the retaining ring 20 such that an exposed retaining surface 21 of the retaining ring 20 is substantially flush with an exposed sapphire surface 41 of the window 40 and the end housing surface 61 of the housing. The window 40, however, may be recessed or even extend in other embodiments.
The invention, now having been briefly summarized, may be better visualized by turning to the following drawings wherein like elements are referenced by like numerals.